Sacrum is a large triangular bone that forms the lower part of vertebral column. It is composed of five sacral vertebrae, which are all fused together to form this triangular wedge shaped bone with forward concavity. The upper border or the base of the wedge articulates with fifth lumbar vertebra. The lower border or apex of the wedge articulates with the coccyx. Laterally, the sacrum articulates with the two iliac bones to form sacroiliac joints.
In children, the sacral vertebrae are not fused. They start fusing together at about 16 years and the fusion is complete by about 26 years.
As described above, the fused sacrum is wedge shaped triangular bone. The base of the wedge is formed by first sacral vertebra. The anterior and upper margin of this vertebra bulges forward as the posterior margin of the pelvic inlet. This bulging margin is called sacral promontory. It is an important obstetric landmark used in pelvimetry.
Sacral canal and foramina:
The vertebral foramina of the sacral vertebrae collectively form a canal called the sacral canal. It lodges the sacral spinal nerves, the filum terminale, and fibrofatty material. The anterior and posterior walls of the canal are perforated by four anterior and four posterior sacral foramina respectively. Through the anterior sacral foramina emerge the anterior rami of sacral spinal nerves. On the other hand, the posterior rami of sacral spinal nerves emerge through posterior sacral foramina.
The laminae of the fifth sacral vertebra fail to fuse in the midline and thus form the sacral hiatus. It is an important site for lumbar puncture.
Ala of sacrum:
On either side of the body of the base of sacrum, there is a large triangular surface called the ala. It supports the psoas major and the lumbosacral trunk.
Median sacral crest:
In the midline of the sacrum on the posterior surface, there is a crest called the median sacral crest. It is often surmounted by three or four tubercles, which represent the rudimentary spinous processes of upper three or four sacral vertebrae.